Fig. 168.
Where a bevel-wheel pattern is too large to be constructed in one piece and requires to be built up in pieces, the construction is as in [Fig. 168], in which on the left is shown the courses of segments 1, 2, 3, 4, 5, &c., of which the rim is built up (as described for spur wheels), and on the right is shown the finished rim with a tooth, c, in position.
The tooth proper is of the length of face of the wheel as denoted by b b′; now all the lines bounding the teeth must converge to the point x. Suppose, then, that the teeth are to be shaped for curve of face and flank in a box as described for spur-wheel teeth in [Fig. 146], then in [Fig. 168] let a, a represent the bottom and b b′ the top of the box, and c a tooth in the box, its ends filling the opening in the box at b b′ then the curve on the sides of the box at b′ must be of the form shown at f, and the curve on the sides of the box (at the point b of its length) must be as shown at g, the teeth shown in profile at g and u representing the forms of the teeth at their ends, on the outside of the wheel rim at b′, and on the inside at b; having thus made a box of the correct form on its sides, the teeth may be placed in it and planed down to it, thus giving all the teeth the same curve.
The spacing for the teeth and their fixing may be done as described for the bevel pinion.
Fig. 169.
To construct a pattern wherefrom to cast an endless screw, worm, or tangent screw, which is to have the worm or thread cut in a lathe.—Take two pieces, each to form one longitudinal half of the pattern; peg and screw them together at the ends, an excess of stuff being allowed at each end for the accommodation of two screws to hold the two halves together while turning them in the lathe, or dogs, if the latter are more convenient, as they might be in a large pattern. Turn the piece down to the size over the top of the thread, after which the core prints are turned. The body thus formed will be ready to have the worm or thread cut, and for this purpose the tools shown in [Figs. 169] and [140] are necessary.
That shown in [Fig. 169] should be flat on the face similar to a parting tool for cast iron, but should have a great deal more bottom rake, as strength is not so much an object, and the tool is more easily sharpened. It has also in addition two little projections a b like the point of a penknife, formed by filing away the steel in the centre; these points are to cut the fibres of the wood, the severed portion being scraped away by the flat part of the tool.
The degree of side rake given to the tool must be sufficient to let the tool sides well clear the thread or worm, and will therefore vary with the pitch of the worm.